Eight children have suffered "potentially serious injuries" after a giant inflatable slide collapsed at a fairground before a fireworks display.
Police declared a major incident at Woking Park in Surrey after the children fell from the slide at about 7.30pm on Saturday.
An estimated 12,000 people had been gathered for a fireworks display organised by Woking District Rotary Club but after the incident, it was cancelled and the park was evacuated to allow the air ambulance to land.
The eight children injured have been taken to major trauma centres, although there is no update on their conditions, South East Coast Ambulance Service said.
A young married couple from Woking, who were taking their nine-month-old son to his first fireworks event, told Sky News they saw the slide collapse before "around 15 to 20 children" fell from it.
The husband, who did not wish to be named, said: "My wife and family were walking towards the slide at around 7.20pm and as we were approaching we saw the side [of the slide] began to buckle about half way or three-quarters of the way up.
"We heard kids screaming and then saw them begin to fall off the side.
"They all collapsed into a pile. We ran over to help.
"Some of the got up and ran away – we don't know but we are worried that they could have had head injuries."
The couple said that the remaining children may have been able to get up but they told them to stay still to avoid exacerbating any potential injuries.
As the husband called the ambulance, his first aid-trained sister-in-law tended to the children and his wife helped to find the children's parents.
The couple told Sky News they believe the slide buckled because too many children were climbing up to the top at the same time and they did not see anyone supervising numbers on the ride.
They both praised the efforts of the emergency services, saying they arrived quickly and that the high number of community police officers were able to help manage the situation.
They said the scene was "very calm".
"It was awful, but it wasn't a disaster where everyone was screaming. There were kids crying but it was calm."
Andy Datson, 23, was at the funfair and told Sky News: "There were lots of kids on the floor net to the inflatable slide.
"I don't want to speculate but the only obvious thing that happened is it gave way on the side and they fell."
Mr Datson said he saw around 40 children on the top of the slide, which he thought was around 25-30ft high, and that he was unable to see anyone supervising it.
"[The children] were doing what they liked, running and jumping when they wanted."
Speaking to Sky News from Woking Park, Adam Morris of Eagle Radio, who was hosting the fireworks event, described the mood as "very sombre… It's very quiet."
He said that after he made the announcement for people to evacuate they did so quickly and calmly.
One of the organisers Michael Holden, from Woking District Rotary Club, said they were "very distressed and upset" by what had happened, adding that their thoughts were "very much with the children affected and their families".
He said that, although it was "too early to speculate" on the cause, the organisation was co-operating with police.
"I don't think crowds were an issue tonight, merely the way that particular ride was operated."
BREAKING NEWS: Woking Park has been evacuated and an air ambulance has just landed to attend the scene of a serious incident. The fireworks have been postponed. pic.twitter.com/njbhmNvldT
— Eagle Radio (@eagleradio) November 3, 2018
The organisers have pledged to conduct "a full review" into the incident and apologised for the "for the short notice cancellation of the Fireworks Extravaganza, caused by a serious accident which occurred in the funfair area during the build up".
Some attendees were criticised for asking for a refund on their tickets within hours of the slide's collapse.
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Woking Park is likely to remain closed tomorrow.
A health and safety executive spokesperson said: "We are aware and are making initial enquiries."